An auxiliary power unit (APU) on an aircraft may be a gas turbine engine configured to produce electrical and pneumatic power. The APU may provide power to start the main engines of the aircraft. The APU may also be used to power systems and devices on the aircraft, such as lights, air conditioning, and flight controls while the main engines are shut down. The APU may be connected to an inlet door, which is configured to be opened or closed and allows air to enter the APU.